Field of the Invention
The invention relates to a hydrostatic positive displacement machine, in particular an axial piston machine with a variable displacement, which to vary the displacement has a pivoting cradle that can be pivoted by a hydraulic positioning device around a pivot axis. The positioning device for pivoting the cradle around the pivot axis has a positioning piston device and, to generate a control pressure that acts on the positioning piston device, has a position-controlled control valve which, to set a pivot angle control value of the pivoting cradle, has setting valve means which are actuated by a setpoint device and comprise feedback valve means which are mechanically coupled with the pivoting cradle for feedback of the current pivot angle value of the pivoting cradle.
Description of Related Art
Hydrostatic positive displacement machines with variable displacement are used as pumps or motors and can be operated in an open or a closed circuit. During the operation of these positive displacement machines, it has proven to be advantageous if the displacement can be varied by varying the inclination of a pivoting cradle on which there is a swash plate on which the displacement pistons of the displacement machine are supported. Hydraulic adjustment devices that can be mechanically, hydraulically, or electrically actuated and controlled are commonly used for this purpose.
To position the pivoting cradle, hydraulic positioning devices have a positioning piston device with at least one positioning piston which can be pressurized with a control pressure, and which is engaged with the pivoting cradle to position the cradle and which thus determines its inclination and therefore the displacement volume. A position-controlled control valve is provided to generate the control pressure exerted on the positioning piston device. The control valve is in communication with a control pressure source and a reservoir and controls the pressurization of the positioning piston compression chambers of the positioning piston device with the control pressure generated by the control pressure source or its decompression (depressurization) to the reservoir. To set a pivot angle control value for the pivoting cradle, the position-controlled control valve has setting valve means which are actuated by a setpoint device, and feedback valve means which are mechanically coupled with the pivoting cradle for feedback of the current pivot angle value of the pivoting cradle.
On positive displacement machines with a position-controlled control valve and a position feedback of the current pivoting angle value, electro-hydraulic positioning devices are known in which electrically actuated control pressure valves, such as proportional pressure reducing valves, are provided as the setpoint device. From an electrical actuation signal, the proportional pressure reducing values generate as a pivot angle control value of the pivoting cradle a hydraulic actuation pressure for a hydraulically actuated slave cylinder, which specifies a stroke to a hydraulically actuated slave cylinder. The slave cylinder is mechanically connected by a rotating lever with the control valve and the pivoting cradle. As a result of the stroke of the slave cylinder, on the control valve (in the form of a longitudinal slide valve), there is an axial movement of the control slide valve (in the form of a longitudinal slide valve member) which forms the setting valve means and connects the positioning piston compression chambers of the positioning piston device with the pivoting cradle by exposing cross sections with the control pressure source and the reservoir so that the pivoting cradle is pivoted around the pivoting axis. The pivot point of the rotary lever in a positioning device of this type is located outside the pivot axis of the pivoting cradle. When there is a pivoting movement of the pivoting cradle and a pivot angle control value specified by the movement of the slave cylinder, by means of the rotary lever (which is coupled with the pivoting cradle) there is thus an axial tracking of the longitudinally displaceable member of the control valve until the exposed control cross sections are reduced in size to the extent that the pivoting movement of the pivoting cradle ends and thus the pivoting cradle is in a positionally balanced control position.
A control valve in the form of a longitudinal slide valve with a control slide valve member which can be displaced longitudinally, is known from FIGS. 5 to 8 of DE 197 53 866 A1. The control valve can be actuated by a rotary lever and the feedback on the position of the pivoting cradle is provided by the rotary lever. Because of the requirement for a rotary lever, however, a position-controlled control valve in the form of a longitudinal slide valve member entails a high cost for components and thus a great deal of construction effort. If the rotary lever is actuated to set the pivot angle control value of a hydraulically pressurized slave cylinder, the number of components and thus the construction effort increases even further, not to mention the fact that the positioning device occupies a great deal of construction space.
To eliminate the expense and effort required for a rotary lever of this type, it is known from FIGS. 1 to 4 of DE 197 53 866 A1 that the feedback of the position of the pivoting cradle on a longitudinally displaceable sleeve of the control valve in the form of a control slide valve is preferable as the feedback valve means. The longitudinal slide valve member of the control slide valve, which forms the setting valve means, is located so that it can be displaced longitudinally inside of the sleeve of the control valve. A component that is connected with the pivoting cradle is engaged in a groove in the longitudinally displaceable sleeve to provide feedback on the position of the pivoting cradle. However, wear can occur between the groove in the sleeve and the component that is connected with the pivoting cradle, which has an adverse effect on the accuracy of the positioning.
DE 196 08 228 B4 describes a positioning device of a positive displacement machine with a variable displacement in which the position-controlled control valve is in the form of a rotary valve. A rotational control valve member of the rotary valve forms the setting valve means and can be actuated to set the pivot angle control value with an electrical stepper motor. The feedback of the position of the pivoting cradle, which is in the form of a swashplate, is provided by a rotating sleeve of the rotary valve in which the rotary slide valve member is located and which forms a feedback valve means. However, on account of the presence of the rotary control slide valve member of a position-controlled control valve of this type (in the form of a rotary slide valve), in which the pivot angle control value is set by a rotational actuation of the control slide valve, it is not possible to actuate the rotary slide valve by a hydraulic actuator pressure generated by electrically actuated control pressure valves instead of by the stepper motor.
An object of this invention is to provide a positive displacement machine of the general type described above but which has a simply constructed and compact position-controlled positioning device, and the control valve of which can be easily actuated by a hydraulic actuator pressure.